This invention relates to devices for automatically controlling the flow of coolant, in accordance with coolant temperature, in a vehicular cooling system, and more particularly to fail-safe thermostats for automotive cooling systems.
In automotive cooling systems, the thermostat is disposed in a junction of at least two passageways, one of which is a passageway to or from a water pump, while another is a passageway to or from a radiator or heat exchanger. A bypass may also be coupled into the junction, to provide a shunt flow around the radiator to allow more rapid warm-up of the engine when starting. A thermostat disposed in the junction, usually in a standardized mount, is accessed through a replaceable dome so that the thermostat may be inspected and replaced if necessary, as in the event of failure. The function of the thermostat is to provide flow control, in response to temperature, so as to utilize the heat exchanger properties of the radiator to maintain the coolant temperature substantially constant in operation. Where bypass operation is used, the bypass should be open only under start-up conditions and should preferably be shut off if the thermostat fails.
Most modern thermostats have brass or similar metal housings of generally circular outline, and incorporate a hollow cylinder disposed about a central axis that encloses one end of a movable central rod. The interior of the cylinder is filled with a thermally responsive wax material which has a substantial thermal coefficient of expansion. When heated, the wax exerts substantial force on the central rod, resulting in a relative displacement. In one type of design, as shown by C. W. Wood U.S. Pat. No. 2,806,375 and U.S. Pat. No. 2,806,376, the central rod is axially movable in and out relative to the housing in response to temperature, and a control device outside the housing may be employed in the principal coolant flow paths so as to provide proportional flow control in accordance with temperature. In another type of thermostat as shown by J. E. Woods U.S. Pat. No. 3,045,918, the variable valve action is provided by a fixed center rod, with movement of an actuator about the rod controlling opening of a valve disk against a tension spring. The mounting for the rod is in the form of an open spider which when open permits flow through the valve.
While earlier types of thermostats are usually reliable in operation, they can nonetheless fail, with what can be catastrophic consequences because of engine over-heating. Parts often stick due to corrosion or the temperature responsive material may leak. Recognizing these factors, various attempts have been made to provide protection against catastrophic operation. Examples are Woods U.S. Pat. No. 3,045,918 and Duprez U.S. Pat. No. 4,537,346. In the Woods patent the valve member is normally urged toward the closure position by a tension spring, but the spring is seated on the valve member in a fusible material which gives way at a certain temperature between the normal range and the level of drastic overheating. The valve member is then acted upon by coolant pressure, and the valve with the attached actuator is freed to open the flow path. This arrangement does not control the final position of the valve and is not configured for bypass operation.
The use of fusible alloys for thermal control of a coolant is also evidenced by the Duprez patent, which, however, is intended to control oil flow through a heat exchanger. In accordance with Duprez an actuator containing an internal actuator rod is disposed in a serpentine oil flow path to function as a spool valve, in dependence upon its position. At the upper end of the structure, an interior spring urges the valve axially downward toward a fully open position, but is restrained by a fusible material outside the cylinder. When the fusible material melts at a particular temperature, however, and flows down into the remainder of the system, the spring is freed to urge the actuator toward a more fully open position, thus preventing overheating. This configuration is specialized, and not amenable to use with automotive thermostats, and further introduces a substantial amount of the fusible material into the coolant flow path.
It is desirable in some configurations to control flow in the bypass path as well as provide proportional control of the valve position in the principal path between the engine passageway and the radiator passageway. In these configurations the bypass path is to be closed off when normal temperature is reached. In all configurations fusible material should not be introduced into the flow path in any substantial quantity. The device should provide substantial protection against sticking, corrosion and other factors that can lead to failure and should be insertable in standardized thermostat mounts.